This invention relates to the field of electromagnetic radiation, in particular visible or infrared radiation, detector networks. The field is limited more particularly to image sensors (ref. [1], [2] of the appended list) composed of a detector circuit interconnected to a read circuit. It relates to a device and method of biasing such photodetectors.
A photodiode is a semiconductor device, which, when illuminated by sufficient energy radiation, outputs a photocurrent by generating electron-hole pairs. Two categories of photodiodes can be produced, according to the type of junction and substrate doping, the N-type junction on P-type semiconductor material and its equivalent, a P junction on N material.
Only the case of N photodiodes on P will be discussed hereafter. All the principles that will be presented are easily transposed to type-P photodiodes on N substrate by a person skilled in the art.
The detector circuit is generally formed of an arrangement of elementary photodiodes implanted with regular spacing according to a matrix of m lines by n columns wherein all junctions are coplanar. Each photodiode is coupled to a preamplifier implanted on the read circuit providing the conversion of the photocurrent output by the detector into a physical quantity compatible with analog processing systems achievable in integrated circuits (current, load, or voltage). The functions implanted on the read circuit also enable multiplexing the information output by each photodiode to a limited number of video outputs. The information output by each photodiode and conditioned by the read circuit analog system corresponds to a picture element or pixel.
The detector circuit can be illuminated either on the side where junctions are made or on the opposite side. The detector circuit is interconnected to the read circuit by means of an adequate method, e.g. microspheres in the case of sensors made by means of a hybrid detector circuit reversed on a read circuit (ref. [3]).
E.g., the principle of photovoltaic detection enables the production of image sensors operating in the visible region spectrum band, or the infrared one (thermal imaging). Spectrum band selectivity is obtained by producing photodiode junctions on a semiconductor material the forbidden bandwidth of which fits the wavelength to be detected.
The invention relates to the method used for biasing the photodiodes of the detector circuit. Hereafter, the review of the state-of-the-art will focus on the issue of biasing photodiodes of such sensors.
First of all, the operating principle of the sensor will be recalled, then the impact of the material, whereon the detector circuit is produced, on controllabilityxe2x80x94i.e. the capacity of imposing a level, here to apply a voltage sourcexe2x80x94photodiode electric nodes.
On the one hand, FIG. 1a represents a look-through cross-section of a junction between a P-type semiconductor substrate 1 and an N-type area 2 producing an N/P junction. The symbolic representation is composed of the symbolic representation of a diode 3, the anode 4 of which is located above cathode 5, so as to show that it is the substrate that is P-type.
FIG. 1b represents the same elements, however, this time, it is substrate 1 that is N-type. A P-type area 2xe2x80x2 is implanted on this substrate 1xe2x80x2 producing a P/N junction. Symbolically, this junction is represented by a diode 3xe2x80x2, the anode 4xe2x80x2 of which is located above cathode 5xe2x80x2, so, as to show that it is substrate 1xe2x80x2 that is N-type.
The current-voltage characteristic of such a junction is represented in FIG. 2. On curve (a), the non-linear characteristic of the ideal junction with zero illumination can be seen: low dynamic impedance when the diode is forward biased, with anode voltage being greater than cathode voltage, and on the contrary, high dynamic impedance when the photodiode is reverse biased with an anode voltage less than the cathode voltage. When the photodiode is illuminated, the current-voltage characteristic, represented by curve (b) is translated vertically by an amount Ip equal to the photocurrent generated by the photodiode. It should be noted that conventionally, the photodiode current-voltage characteristics are represented in conventional quadrants and not with the actual current and voltage signs.
The schematic diagram of a sensor is represented in FIG. 3, it corresponds to the cross-section of a matrix sensor, normal to the layer planes, following one of the directions of the lines and columns of the sensor matrix.
This diagram illustrates the case of a hybrid detector circuit 17, reversed on a read circuit 20 as mentioned, e.g., in document [2]. The N photodiodes of the row corresponding to the cross-sectional plane are marked D1 to DN, their anodes A1 to AN and their cathodes K1 to KN. The photodiode anodes of detector circuit 17 are connected to the inputs E1 . . . EN of read circuit 20. Continuity between detector 17 and read 20 circuits is provided by a vertical connection, e.g. of the indium microsphere type 21.
The read circuit preamplifiers 20 are numbered from PA1 to PAN.
The imaging process of this pixel row is the following one:
1. biasing the photodiode during image sensing so that it delivers a photocurrent;
2. processing the current output by the photodiode by means of preamplifiers;
3. multiplexing the output signal of each preamplifier to a video output.
The process is repeated at frame rate.
In practice, each photodiode is biased in the reverse part of its characteristic, at a voltage where the intensity of its current with zero illumination is relatively low in comparison with its photocurrent intensity. Controlling the difference of potential between the anode and cathode of each photodiode is therefore decisive for the operation of the detector circuit.
Controlling the potential of each anode is provided by the preamplifier input (e.g., virtual ground of a differential amplifier). On the other hand, the cathodes of each photodiode cannot be controlled individually. In fact, they are short-circuited by the semiconductor material where the junctions are made. Therefore, the cathodes K1 to KN can only be controlled indirectly, via a single electric node of the detector circuit identified as KCxe2x80x94for common cathode.
The electric characteristics of the layers composing the slice whereon the detector circuit is produced will determine the resistor for accessing the cathode of each photodiode. A schematic cross-section of these slices is represented in FIG. 4. We can distinguish between three categories of slices:
1. the so-called solid substrate ones, represented in FIG. 4-A;
2. the so-called epitaxial substrate ones, represented in FIG. 4-B;
3. the so-called insulating substrate ones, represented in FIG. 4-C.
The solid substrate of FIG. 4-A is composed of a single layer 4 for the whole slice thickness. Slice resistivity xcfx811 is uniform and suitable for realizing high-performance photodetector junctions.
The epitaxial substrate, FIG. 4-B, is a dual layer one 7, 8. The photodiode junctions are made in the upper layer 7 of reduced thickness and resistivity xcfx812 suitable for producing photodiodes. The bottom layer 8 is made of the same material. It is very thick and its resistivity xcfx813 is very low, for minimizing the resistor accessing the junction cathodes.
The top layer 9 of an insulating substrate 10, FIG. 4-C, has thickness and resistivity characteristics that are close to that of the epitaxial substrate. The base 10 thereof is also very thick. It can be produced by stacking up various materials, but at any rate, it acts as an electric insulator.
The detector circuits operating in the visible spectral range are produced on solid or epitaxial, or even insulating (ref. [1]) silicon substrates. Those operating in the infrared spectral range are rather produced on insulating substrates (e.g. the HgCdTe semiconductor material on a CdZnTe insulator of ref. [2]). At any rate, photodiodes are coplanar.
The common cathode of the photodiodes of detector circuits 17 made on the solid or epitaxial substrates can obviously be controlled by providing electric continuity between a voltage source 11 and a contact 12 made on the opposite side 13 of the detector circuit junctions 1, 2. However, this solution represented in FIG. 5 is not always applicable.
First of all, the technological manufacturing method must incorporate additional steps for producing the ohmic contact 12 on the opposite side 13 of the junctions 1, 2.
Next, for sensors illuminated through substrate side 13, the loss of light flow at the ohmic contact 12 of substrate side 13, or even its attenuation throughout the passage can turn out to be prohibitive.
Finally, applying a voltage source to the opposite side 13 of the junctions (1, 2) can be done by means of a soldered conductive wire 14, also represented in FIG. 5, but the following is required:
the circuit supports the mechanical and thermal constraints induced by the interconnection method employed,
the detector circuit is surrounded by an area the dimensions of which are compatible with this methodxe2x80x94no deterioration of the electro-optical characteristics of the photodiodes located nearby, no wire shadows on the optically sensitive area.
Due to these reasons, the controllability of the common cathode is often provided for detector circuits produced on solid and epitaxial substrates by using the technique that has to be implemented for insulating substrates and which will be described hereafter with reference to FIG. 6.
In this case, the detector circuit manufacturing method incorporates technological steps allowing to produce, in addition to N-type junctions 1, 2 on the P-type semiconductor material, an ohmic contact 12 on this material.
A schematic cross-section of such a detector circuit 17 is represented in FIG. 6. The N+ areas 2 stand for the photodiode anodes. The P++ area stands for the ohmic contact 12 on the type-P material.
At the sensor level, electric continuity between the ohmic contact 12 of the common cathode and its power supply is provided via the read circuit, the interconnection between the ohmic contact 12 made on the detector circuit 17, and the read circuit is provided by the same interconnection method as that used for linking each anode to its preamplifier, e.g. an indium microsphere 21.
As can be seen, this technique of biasing the common cathode via the side 15 containing junctions is applicable to the circuits produced on the 3 substrate categories.
In addition to the fact that it avoids a connectivity operation on substrate side 13, this method releases the opposite side 13 of the junctions from any electric constraint related to the common cathode. Thus it is possible to optimize the optical response of the circuits illuminated through their substrate, e.g. by removing all or part of the substrate base.
Whatever the nature of the substrate used for manufacturing detector circuit 17, controlling the potential of the common photodiode cathode by means of an ohmic contact 12 on the semiconductor material, produced on the side 15 containing the coplanar junctions of the detector circuit turns out to be a good compromise between:
the difficulties of manufacturing the detector circuit 17
the sensor""s electro-optical performance
the constraints of assembly in boxes.
However, this control method has some disadvantages that will be approached below.
As is the case with most detector circuits, we will assume that the ohmic contact 12 of the common cathode is implanted at the periphery of the area occupied by the photodiodes.
The electric diagram of FIG. 3 must be modified as indicated in FIG. 7 to take into account the phenomena induced by the finite resistivity value of the semiconductor material whereon junctions are produced:
the inter-cathode resistor (RIK) reflects the equivalent electric resistance between the cathode of a photodiode and that of the one associated therewith,
the resistor for accessing the common cathode electric node (RAKC) represents the electric resistance of the area separating cathode KN from the ohmic contact 12 of the common cathode KC.
Each photodiode behaves like a current generator IAK1 . . . IAKN. The current generated for each photodiode crosses resistors RIK separating the photodiode from resistor RAKC and resistor RAKC itself so that it is crossed-by the sum of photodiode currents.
This one-dimensional diagram is sufficient for demonstration purposes. Generalizing it to a two-dimensional detector circuit results in a two-dimensional array of inter-cathode electric resistors, each photodiode being associated with the photodiodes implanted on its four sides and with the resistors for accessing the common cathode, which vary depending on how the P++ areas are distributed over the photodiode periphery on one side, two adjacent sides, two parallel sides or else on the four sides of the photodiode matrix.
Such a photodiode matrix is schematically represented in FIG. 8 and will be described hereafter. This matrix 16 has a set of junctions 1, 2 arranged in a matrix. A P++ contact 12 composing a closed line surrounds the matrix set 16 of junctions 1, 2. The limits 30 of detector circuit 17 are represented by a closed dotted line 30.
Around its operating point, each photodiode Dn can be considered equivalent to a current generator with a value IAKN, as explained above with reference to FIG. 7. Such a representation is the object of FIG. 9.
The electric diagram of FIG. 9 clearly highlights that the currents output by the photodiodes are summed as the electric node KC is approached. The current flow in resistors RIK causes a potential drop that increased when moving away from node KC. Therefore, the potential of each cathode KN is not equal to the voltage applied to the node of the common cathode KC.
This biasing technique introduces a space variation of the voltage applied to the photodiode cathode: all other conditions being the same, the photodiodes are not biased with the same potential difference between anode and cathode.
From a certain threshold on, the space variation of the photodiode operating point has the effect of reducing sensor performance because the optimal biasing range is reduced thereby.
The depolarization space variation amplitude is all the greater since the number of sides whereon the P++ ohmic contact 12 can be implanted is reduced.
The response to localized overillumination in space is a decisive criterion for an image sensor. Ideally, the photodiode response of the overilluminated area must not modify the response of the photodiodes located outside of this area.
Given the electric diagram of FIG. 9, it is clear that the overillumination of a photodiode located in the middle of the detector-circuit will cause a high current to flow through resistors RIK and RAKC that are separating it from electric node KC. The potential drop induced by this current flow will modify the cathode. potential of the photodiodes located upstream and downstream of the overilluminated photodiode. As they are no longer biased at the same operating point, these photodiodes will output a different intensity current if their dynamic resistance is not infinite, while the illumination they are receiving has not changed.
With this method of biasing photodiode cathodes, the electro-optical response of the overilluminated photodiodes introduces a variation in the electro-optical response of all or part of the other photodiodes of the detector circuit while their illumination has not varied. This phenomenon can for instance cause false alarms.
Producing a P++ contact near a N+ photodiode must comply with a set of design rules. In practice, it is not possible to design such a contact between the photodiodes of a detector circuit the photodiodes of which are designed with reduced spacing.
This difficulty could be obviated by replacing one or several photodiodes with a P++ contact. However, this would have the effect of increasing the number of the sensor""s blind pixels.
Therefore, the P++ contact 12 of the common cathode is generally implanted on the periphery of the detector circuit, as represented in FIG. 8. The overall size of the detector circuit 17 is thus significantly increased with respect to the dimensions of the sensitive area 16 occupied by the photodiode matrix.
This is due to the fact that on the one hand, a P++ ohmic contact 12 is normally designed in ring-shape around the photodiode matrix 16 in order to minimize the space variations of the photodiode series resistance, and on the other hand, that it is necessary to keep a minimum distance between the P++ ohmic contact 12 and the N+ areas of the photodiode junctions (2, 1).
Therefore, this method does not allow to maximize the filling factor of the detector circuit 17 with its photodiodes, which translates into material loss for a given number of pixels, or else a decrease of the number of pixels for a given detector circuit size 17.
Although it is incorporated in most detector circuit manufacturing methods, producing a P++ ohmic contact 12 does not go without problems.
First of all, it significantly increases the number of manufacturing stepsxe2x80x2. Indeed, it is necessary to produce P++-type areas, then to pick up ohmic contacts in these areas.
Furthermore, the technological methods implemented for producing ohmic contacts and interconnections must be compatible both with P++ areas and with N+ areas. The choice of the technological methods is therefore more limited than in the case where it is only necessary to provide electric continuity with N+ areas.
In conclusion, the necessity of producing the ohmic contact required for biasing the common cathode results in a more complex manufacturing method than that required for mere photodiode production.
The invention eliminates the need to keep the ohmic contact 12. Thus, a set of coplanar photodetectors can be obtained, together composing an image sensor circuit, each photodetector composing in general one image pixel, wherein the value of the signal output by one of the photodetectors is not modified or not much modified by the value of the signal output by other photodetectors of the sensor. It aims at obtaining a detector circuit with greater density per unit area, i.e. with a greater number of photodetectors per unit area of a detector circuit. Finally, it aims at simplifying the manufacturing of a detector circuit.
For all these purposes, the invention relates to an image sensor comprising:
a detector circuit composed of a first set of coplanar photodiodes carried by a substrate, each photodiode having an anode and a cathode,
a read circuit composed of a set of elementary read means, with the detector circuit and the read circuit being interconnected so that each elementary read means is coupled to the anode of a photodiode,
a means for biasing photodiodes by creating a potential difference between each photodiode anode and cathode,
the image sensor being characterized in that the substrate of the detector circuit has a second set of diodes that are coplanar and located in the same plane as that of the first set, each diode of the second set having a cathode and an anode, each diode of the second set being associated with at least one photodiode of the first set, each anode of the diodes of the second set being connected or can be connected through the read circuit to a first voltage source (Vapol) reverse biasing the diodes of the first set, and in that each anode of the diodes of the second set is connected or can be connected through the read circuit to a second voltage source (Vkpol) forward biasing the diodes of the second set, with the voltage for biasing the photodiodes of the first set thus being controlled by controlling the voltages applied to the anodes of the first and second set.
Voltage Vapol, which is applied to the anodes of the photodiodes or detector diodes for biasing them, is the detector voltage. Voltage Vkpol, which is applied to the anodes of the diodes of the second set or the control diodes for biasing them, is the control voltage. The control voltage applied to an anode of a control diode forces a potential onto the cathode of the neighboring diodes.
Thus, it appears that due to this way of controlling the cathode of the detection biased diodes or junctions of a detector circuit, the impedance between a cathode of a detection biased junction and the cathodes nearest to the control diodes is essentially the same.
It should be noted that so far, it has been considered implicitly:
1. that the substrate has omnidirectional homogeneity,
2. that when there are several cathode voltage application points neighboring a photodetector diode, the photodetector current of this diode is distributed in parallel among the photodiode and these various points.
The result is that the influence of one diode or a group of neighboring detector diodes blinding each other is limited to one portion of the detector delimited by a line joining the reference voltage application points that are closest to the junctions located at the limit of the detector""s blinded portion.
It will be possible to perform reference voltage application through conductor feedthroughs of the read circuit, directly applied to the detector circuit via forward biased detector circuit junctions in order to obtain low junction impedances. These diodes can be diodes solely dedicated to biasing.
It will appear below that, in the preferred embodiment, the detector circuit photodetector diodes are used for applying the reference voltage, which are immediately neighboring detection biased diodes.
Thus, the potential of the cathode of a detector circuit photodiode is controlled by means of photodiodes associated therewith. For this purpose, associated photodiodes just have to be biased for their forward characteristic, at the point where they have very low dynamic resistance. Potential transmission is provided through the read circuit, contact between the read circuit and the photodiode being provided by a known means.
The invention also relates to a method of biasing photodiodes of an image sensor detector circuit, the detector circuit having, on the same substrate, two sets of coplanar diodes, a first one comprising coplanar photodiodes and a second one comprising coplanar control diodes, each diode of each of the sets having an anode and a cathode, and one control diode being associated with at least one photodiode, the method being characterized in that a voltage reverse biasing the diodes of the first set is applied to the anodes of the first set diodes, and in that a voltage forward biasing said diodes of the second set is applied to the anodes of the second set diodes.